The magnetic field coil for use in NMR-CT apparatus is required to generate a uniform magnetic field, having an intensity of at least 0.1 to 0.2T and a uniformity as high as 100 ppm, within a spherical magnetic field space in diameter of about 40 cm that is provided at the center of magnetic field coil, and in which a human body is set. Moreover the coil is also required to keep the variation of the intensity of the magnetic field during operation as low as 10 ppm or less, in order to prevent the disturbance of the tomographic image during the pick-up time of the image (hereinafter called the operation time), which may last as long as 10 to 20 minutes. Moreover, any expansion and compression or deformation of the coil due to a temperature change of the coil has a large influence in changing the magnetic field during the operation. Therefore it is usual to include both a cooling structure which keeps the temperature change of the coil from the original temperature set to .+-.0.1.degree. C. or less, and also preheating apparatus that maintains the temperature even after the tomographic image pick-up operation ends to keep short the time for stabilizing the coil temperature when image pick-up operation is started again.
FIG. 4 schematically illustrates a prior art example of the structure of the cooling apparatus and the preheating apparatus for a uniform magnetic field coil.
In this figure, reference numeral 1 denotes a uniform magnetic field coil consisting of two pairs of Helmholtz coils 2, where a pair of liquid cooling plates 3 are respectively provided closely to both sides of a ring-shaped coil having a rectangular cross section and the coils 2 are connected in series with a DC power supply 4 for excitation. Numeral 10 denotes a cooling apparatus using a cooling liquid 100, such as water or oil, etc. comprising a flow cooling device 5 consisting of a compressor 5A, a condensor 5B, a radiator 5C and a capillary tube 5D, a heat exchanger 6 which is cooled by the flow cooling device, a circulation pump 7, a back pressure pump 8, and a liquid tank or reservoir 9 which also works as a venting tank for the cooling liquid and houses a float valve 9A and a temperature sensor 9B. Cooling apparatus 10 communicates with the cooling plates 3 of the coils 2 by way of a regulator valve 13B consisting of a pair of valves that operate in synchrony, and a cooling liquid path 13A. The cooling apparatus also cools a rectifying element of the DC power supply 4 through the cooling liquid path 14A. Moreover, numeral 24 denotes a preheating apparatus comprising a temperature sensor 24A which senses temperature at the inlet port of the cooling liquid and a control circuit 24B which receives an output signal from the temperature sensor 24A and generates a signal for controlling the output current of the DC power supply 4.
In apparatus constituted as described above, temperature control (cooling) of the uniform magnetic field coil 1 which is generating the uniform magnetic field 11 is carried out as follow: First, both of the regulator valves 13B are opened and the bypass valve 14C is closed. Then the cooling liquid which has been cooled to a predetermined temperature T.sub.0, for example, to 20.degree..+-.0.1.degree. C., by starting the pumps 7, 8 and flow cooling device 5, is made to circulate into each cooling plate 3 of the coil 1 and the power supply 4. Also a predetermined excitation current I.sub.0, is supplied to the coil 1 by starting the power supply 4. In this case, since the thermal resistance between the coil and cooling liquid is smaller than that between the coil and ambient air, the temperature of coil 1 is little influenced by the ambient temperature and it can be kept constant to a temperature T.sub.1, for example, to 40.degree. C., where the heat generated in the coil is equilibrated to the heat eliminated by the cooling liquid 100. In other words, a stabilized tomographic image can be obtained by limiting the temperature of cooling liquid 100, which is controlled by an output signal of the temperature sensor 9B, to a variation of 0.1.degree. C. or less during the operation time required by the NMR-CT for a tomographic image. Next, the regulation valves 13B are closed when the tomographic image is completed, and only the power supply 4 is cooled by adjusting a bypass valve 14C. The coil 1 is then set to self-cooling operating condition and the power supply 4 is made to control the preheating operation by the preheating control apparatus 24 comprising the temperature sensor 24A and control circuit 24B. In this case, when the setting temperature of control circuit 24B is set to a temperature which is almost equal to the operating temperature T.sub.1 of coil 1, the output current of the power supply 4 is reduced when the temperature of cooling plates 3 comes close to the operation temperature T.sub.1, and the temperature of the coil can be stabilized and maintained at a temperature near the operation temperature under the condition that the heat generated by the coil is balanced with the heat radiated by natural cooling.
However, when coil 1 is enclosed within an air-conditioned chamber having a cover (not illustrated) for temperature control as is usual, a temperature difference in the vertical direction is generated in the ambient temperature within the cover. Accordingly, the coil in the self-cooling condition is influenced by the ambient temperature, resulting in the problem that the temperature is higher at the upper part of coil than at the lower part. As a result the coil is thermally deformed due to the generation of thermal stress within the coil. Under this condition, when the operation is restarted by closing valve 14C, opening regulation valves 13B, and applying an excitation current I.sub.o to the coil 1 by changing the preheating control operation of power supply 4 to its normal operating condition, since the cooling liquid 100 which is cooled to temperature T.sub.0 by operation of the cooling device 10 even during preheating control operation, changes its flow path from the bypass valve 14C to the cooling plates of the uniform magnetic field coil 1 through regulation valves 13B the coil can be cooled immediately. Nevertheless it is found that a stabilization time (waiting time) of at least 45 minutes or more is actually required until the tomographic image is stabilized, even though it is estimated that the temperature of the coil is equilibrated to the operation temperature T.sub.1 within a shorter period. Therefore shortening of the stabilization time and improvement of the working efficiency of NMR-CT are desirable.
The present invention has been developed in light of the aforementioned background and the object of the present invention is a uniform magnetic field coil comprising a novel preheating apparatus of simple structure to reduce the stabilization time.